1. Field of the Invention
The present invention relates to a picture signal generating apparatus having a linear image sensor.
2. Related Background Art
In order to generate a picture signal upon reading of a two-dimensional image in a conventional system, a linear image sensor (e.g., a CCD line sensor) having a plurality of photoelectric transducer elements aligned in line along one direction is moved in a direction perpendicular to an arrangement direction of the transducer elements to read out an image. For this purpose, the intensity of light transmitted through or reflected by an original is kept constant to obtain a normalized picture signal having an accurate signal level corresponding to the density of the original image.
If the speed of the CCD line sensor is irregular, a picture signal representing an image distortion in a direction corresponding to the moving direction of the CCD line sensor is generated. More specifically, if the speed is reduced, the image is prolonged. However, if the speed is increased, the image is shortened.
In order to eliminate the image distortion along the scanning direction, a displacement of the CCD line sensor must be spontaneously detected. For example, an image read instruction pulse may be generated whenever the CCD line sensor is fed by a displacement corresponding to one pixel.
However, if the image read instruction pulse is generated for every predetermined displacement of the CCD line sensor so as to eliminate the image distortion caused by irregular speeds along the scanning direction, the image distortion can be eliminated. However, an interval of the image read instruction pulses is not kept constant. The exposure for determining the level of the picture signal varies according to the pulse interval. The picture signal from the CCD line sensor cannot be normalized by constant control of the intensity of illumination light. As a result, the signal levels vary according to changes in exposure caused by irregular scanning speeds.